Length or distance measurement techniques, once limited to mechanical devices such as tapes and sticks of known length to stretch across or otherwise traverse a distance to be measured, have evolved. Newer techniques include the transmission of a pulse of energy (“transmitted pulse”) at a time of transmission (“TOT”) to a target at the far end of the distance to be measured. A return pulse of energy (“return pulse”) reflected from the target is received at the point of transmission at a time of arrival (“TOA”). A time difference between the TOA and the TOT is calculated. The round trip distance between the point of transmission and the target is then calculated as the time difference multiplied by the speed of transmission of the pulse of energy through a medium such as air. A sonic pulse is transmitted at the speed of sound through the medium across the distance, for example, and a light pulse by the speed of light through the medium. The latter case is often referred to as LIght RaDAR or “LIDAR,” and is referred to by that acronym herein.
FIG. 1A is a block diagram of a prior-art LIDAR return signal processing apparatus 100. The apparatus 100 is enabled following transmission of the transmitted pulse and detects electromagnetic energy at wavelengths including those associated with the transmitted pulse at a photo-detector 105. The photo-detector 105 converts the light energy into an electrical signal that is amplified and otherwise analog processed in an amplifier 107. The processed signal appears at an output 110 of the amplifier 107 and is referred to hereinafter as the “return signal.”
FIG. 1B is a prior-art waveform diagram of an example LIDAR return signal 113. The timeline 114 associated with the waveform diagram includes a partial time segment 115 during which the outgoing pulse (not shown) is transmitted at t=0. Referring back and forth between FIG. 1A and FIG. 1B, a comparator 118 compares the magnitude of the return signal 113 to a threshold reference voltage 125. The threshold reference voltage 125 is generated by a threshold reference voltage source 120 and is preset to distinguish noise from a return pulse 135 corresponding to a target object at the distance to be measured.
An output 140 of the comparator 118 is triggered when the magnitude of the return signal 113 exceeds the threshold reference voltage 125. A clock in a time measurement unit (TMU) 145 is started at outgoing pulse TOT, t=0. The clock is stopped at the TOA 150 of the return pulse 135 corresponding to the positive-going threshold crossing event. A processing circuit 155 converts the TOA 150 to a distance-to-target 160 from the round trip distance 165 based upon the speed of light through the traversed medium as described above.
FIG. 2 is a block diagram of a prior-art LIDAR return signal processing apparatus 200. The LIDAR apparatus 200 includes a photo-detector 105 and an amplifier 107 to generate the return signal 113 as described above with reference to the apparatus 200. However, the apparatus 200 also includes a fast analog-to-digital converter (“ADC”) 205. The ADC 205 continuously generates digital samples of the return signal 113 and presents the samples to a processing apparatus 210. A processing apparatus 210 performs various signal analysis operations on the digital samples. The signal analysis operations enable the LIDAR apparatus 200 to better distinguish the portion 135 of the return signal 113 corresponding to the target object from other portions of the return signal 113 and to obtain and deliver from the portion 135 the distance-to-target 160 at one or more output terminals.
Generally speaking, LIDAR return signal processing apparatus such as the apparatus 100 utilize lower-cost components and consume relatively less power than full return signal digitizing and processing LIDAR apparatus such as the apparatus 200. However, the processor 210 of the apparatus 200 has access to samples from the complete LIDAR return signal and can statistically process the samples using digital signal processing (“DSP”) techniques.